1. Field of the Invention
This invention generally relates to electronic components that are secured to electronic carrier substrates. More specifically, the invention relates to providing such components with expansion joints to reduce the effect of thermal differential between the component and the carrier substrate, which for example may be a printed circuit board.
2. Background Art
Decreasing packaged circuit board size, decreasing packaging costs, and increasing component-packaging density are ongoing goals of the computer industry, and specifically the substrate packaging industry. Accordingly, various components are typically connected to the carrier substrate, for example printed circuit boards, through an electronic connection made by a conductive material, such as solder. Some components connect directly to the carrier substrate, such as capacitors, resistors, CPUs and the like. Other components detachably connect to the carrier substrate through various connectors, which are connected to the carrier substrate. Connectors come in a variety of sizes and mount to carrier substrates to enable various memory modules and input output devices to electrically connect to the carrier substrate. Example connectors include Dual In-line Memory Module Sockets (“DIMM Sockets”) and card edge connectors for secondary boards.
The effectiveness of the components that connect to the carrier substrate depends largely on the integrity of the electrical connection between the carrier substrate and the particular connector or component. Two primary techniques to attach components to carrier substrates are Through Hole Mount technology (“THM”), and surface mount technology (“SMT”).
THM has been a common way of attaching components, especially connectors, to carrier substrates for over forty years. In THM components, the component leads consist of individual pins that engage a corresponding electrical interface pattern, which are holes in the carrier substrate that form a hole pattern. THM technology, however, limits the density that components can be placed on a carrier substrate because the holes penetrate all layers of the carrier substrate, which in turn restricts or blocks routing channels on every layer.
SMT is a newer technology. SMT differs from THM in that the electrical interface pattern on the carrier substrate are not holes, but rather consists of pads that reside on the surface of the carrier substrate. The component, then, sits on top of the carrier substrate with its component leads in contact with or in close proximity to the carrier substrate electrical interface pattern. This maximizes the available routing space on the carrier substrate and allows denser packing of components.
Currently, both THM and SMT components are typically fixed to the board through a soldering process. THM components typically go through a wave soldering process, which is well known to those skilled in the art. Briefly, in a wave soldering process, THM components are placed on the carrier substrate, the carrier substrate enters the wave solder machine, encounters a preheat region, and is passed over the top of a molten solder wave. When the solder wave contacts the portion of the component leads protruding through the electrical interface pattern holes, the solder is drawn up into the hole and the component lead via capillary action. As the board moves beyond the solder wave, the temperature is reduced and the solder solidifies, securing the connection between the component lead and the electrical interface pattern hole in the carrier substrate.
SMT components are typically connected to the carrier substrate through a solder reflow process. The solder reflow process is also well known in the art of carrier substrate packaging. A conductive material is placed on the carrier substrate at designated spots to which the leads of the component are to attach. The component is placed onto the carrier substrate with conductive material (e.g. solder) residing between the component leads and the electrical interface pattern of the carrier substrate. The carrier substrate then passes through a reflow oven where it encounters a profile of gradually rising temperature, reaching a peak temperature above the solder reflow temperature where the conductive material melts and makes the electrical connection. The process is concluded with a cool down period where the conductive material solidifies. The solder reflow process is more desirable than the wave solder process in that it is generally more efficient, environmentally friendly, and cost effective.
A problem exists, however, with both the wave solder process and the solder reflow process. Varying Coefficients of Thermal Expansion (CTE) between connectors and Printed Circuit boards (PCB's) create high stresses and unmanufacturable solder interfaces on long Surface Mount Technology (SMT) parts. Current solutions include trying to match connector CTE with PCB CTE. The costs of doing this are high and not possible in many cases. During the solder reflow process, both the PCB and connector are heated in a convection reflow of vapor phase oven. As the PCB cools, the solder solidifies and then the PCB and connector continue to cool down at different rates. CTE mismatches between the board and connector create stresses in the solder joints and bow both the connector and board which could lead to reliability problems and low manufacturing yields. This mismatch of CTE will cause bowing of the connector, possibly lifting the connector contacts which in turn will result in missing or insufficient solder. If the connector remains bowed, plugging problems may occur on the separable interface, yet another reliability concern. The amount of bowing will depend on connector length (CTE differential), board thickness, reflow profiles, and Tg (glass transition temperature) of the board.
Also, when a lot of the connectors are placed on a PCB, the board itself can bow as the solder solidifies and the glass transition temperature is below the solder solidification temperature. This, too, becomes a reliability and plugging issue, since the DIMM (for example) will not seat fully into the connector.